Apparatus for measuring light.



' PATENTED MAY 26 W. J. HAMMER.

APPARATUS FOR MEASURING LIGHT.

APPLICATION FILED OCT. 31,1906,

worthless.

' amp e, wit

WILLIAM J, HAMMER,

or NEW YORK, N. Y.

APPARATUS FOR MEASURING LIGHT.

Specification of Letters Patent;

' Patented May 26,, 1903.

Application filed October 31, 1906. Serial No. 341,425.

To all whom it may concern:

Be it known that 1, WILLIAM J. HAMMER, a citizen of the United States, residing in New York city, in the State of New York, have in: yente'd certain new and useful Improvements in Apparatus for Measuring'Light, of which thefo owing is a specification.

My present invention relates to the art of measuring light, and particularly to an improved a paratus for use in that art, by which lig ts may, be compared without introducin-g into the result the inaccuracies'due to the physiological factor involved in employing the eye as a standard in the compari son.

men use and above indicated are well known; in many cases differences amounting to as much as ten per cent. will exist between the estimates made by different observers with the best scientific means of comparison. Furthermore, the colors of the lights to be compared are of great importance, inasmuch as not only do the colors differ in the intensity of the effect roduced u on the eye of an individual but t ere is mar ed variation be: tween difl erent persons inthis res' ect; so that by some observers two lights of t e same color may be compared with considerable accuracy, but a change in the color of'the standard will render the determination In addition the practical im-, possibilit of maintaining a standard light which wi not vary from time to time with the pressure of the atmosphere and its specific composition at the placeof test ,and with the purity of the materials used to produce the flame (a flame bei-n usually employed for this purpose) affects t e precision of comparison to a greater or less extent.

The common standard of comparison is the well known pentane lam ;-the color of this standard is towards the re end of the spectrum, and to most eyes it shows decidedly pink. Hence when an attempt is made to com are a ellow incandescent lamp, for exthe standard, the results obtained from different observers are often so discordant as tomake them of little use; it

has therefore become a practice to have a single observer compare a number of incandescent lamps withthe selected standard with great care, so as to make them secondary The inaccuracies of the method now in comstandards, which when burned at pro er voltage will give for a considerableperio a substantially unvarying li ht. Obviously,

however, this introduces sti lfurther inaccuracies, since the lamps will, in s ite of all pos-' sible care, differ among themse ves, and comfatigue and with physical condition, as'is well understood.

The difficulties above set forth, I obviate by employing the apparatus which I'am now to describe, which consists in brief of the combination of means for causing the light to be measured to vary a physical quantity capable of accurate determination, and means for measuring the chem e in the 'uantity due to the action of the lig t. While a number of different ways may be em loyed, I refer for the purpose named an e ectrical evice or combination of electrical devices which cause an electric current to vary in accordance with the intensity of the light to bemeasured. The variations of electrical'currents may be measured with great accuracy intermsof an absolute standard, and the measurements are not specifically affected by the personal equations of the observers, so that the same results may (within an inconsiderable rangeof error) be obtained by different observers or by the same observer at different times To carr out the invention, therefore, I cause an e ectric current, preferably small, to flow in a circuit comprising the source of current, a measuring instrument and a circuit, a art of which at least,'is composed of a su stance responsive in its electrlcal condition to light; for this substance, the metal or element selenium, the resistance of which is considerable and also varies over wide limits in response to light, is the one which I The method of operation, which I believe to be broadly new, is not claimed herein, be-

ing described and claimed in my'application for Letters-Patent, Serial Number 341,426, filed October 31st, 1906. j

The accompanying drawings illustrate an embodiment of my invention.

In these, Figure 1 is the electric device or combination which I have briefly indicated in my statement of invention. Figs. 2, 3

' and 4 show respectively a means of utilizing the varlations of-am lltude of sound-waves,

and a modification t lereof.

in Fig. 1, A is the battery or other source of current which is to be measured and variations in which are taken as indicative of light intensities. The battery is arranged in circuit with a selenium cell B, which is preferably of that form in which the solenium, after being coated upona suitable conducting sup ort, such as a coil of nickel wire, is sea ed in a tube of preferably as nearly perfect a vacuum as can be obtained, the ends of the conducting wire being passed through the glass by means of platinum tips or otherwise; since the current is small and no great heat is evolved, platinum is not necessarily used. (1 is a measuring instrument, in the case illustrated a voltmeter, responsive to small variations of pressure. I have not illustrated any current-controlling means, since obviousl such devices may be employed as the expert may desire, and customarily the current will be regulated by the usual resistances and switches in ways well understood. Preferably the current through the cell should be of as low a value as can be conveniently maintained constant, for several reasons, one of which is that a large current evolves so much heat that the cell is subjected to alternate expansions and contractions which are detrimental; but the most important reason is that for very small currents a proportionately larger variation of voltage is caused by the action ofthe light upon the cell, rendering the device more sensitive and accurate. Upon a suitable table D, is arranged the lamp E, to be tested. At F, I have indlcated a screen having a hole f which may be closed .by a slide F. In practice it is referable to lace the seleniumcell in a lig t-tight box 1 (shown partly broken away) having free ventilation, so that the indications may not be affected by exterior light; and inasmuch as the effect to be observed is not detectable by examination of the cell, it is only necessary to open the The method of oper-- box at long intervals. ating the arrangementthus described is substantiall as follows: A standard lamp of any desired construction, such as the-pentane lam referred to in the previous description, is p aced in pro or position adjacent to the a paratus and ighted, being screened comp etely from the cell; a small current, preferably a small fraction of an ampere, is

then caused to flow in the circuit including the cell, and after it has attained a stead value, the fall of potential around the cell is measured by the voltmeter. After this the light of the standard lamp is allowed to fall upon the cell, and the change in the resistance of the circuit (as indicatedby the change in the drop).-caused by the action of the light upon the selenium is measured and recorded; this then becomes a constant parisons of (ill'erent cells to guard against' change.

In Figs. 2 and 3, I show a second means of practicing the method which i have indicated. This consists of a siren-disk G of well known construction in which the rows of holes 9 g bear to each other certain definite relations, such as the notes of the scale. 0 posite to, and rotating with this disk (the perforated disk being shown detached for clearness of illustration) is a disk II bearing upon its surface conductors coated With selenium, one for each of the rows of holes g. The ends of the conductor go to collector rings rotated with the shaft, the whole being driven at constant speed. In the circuit with the collector rings and source of current supply is a coil 1, disposed so as to affect a tuning fork K provided with a pointer 70, the amplitude of vibration of which is recorded upon the revolving cylinder L, in the manner of a chronograph. The rowsof holes in the siren-disk may conveniently be in the relation of the notes of the octave of a tempered scale, and may each be provided with a similar tuning-fork, whichat the normal rate of rotation will respond to its ap ropriate tone; these being all substantially ike the fork K and arranged in a similar way, are not illustrated. Such an organization will enable he observer to conduct a number of tests at the same time, when desired.

The apparatus thus described being brought up to normal speed, which should be maintained as nearly constant as possible,

the tuning-fork K will begin to vi )rate as soon as the standard lamp is turned on, and

the amplitude of its vibrations will be re.-

' corded upon the cylinder L; upon turning off sss,s01

- A telephone N may be connected in circuit if desired, either directly or inductively, and the note of the fork will'then sound in the ear, of the observer; an operator with a good musical ear may reach a very close appointed out with respect to visual photometers. 1

In Fig. 4, I indicate a construction which 7 isr-chea er and yet substantially effective.

20, manipulationsmay be almost indefinitely In this gure the cells B are arranged upon a stationary support and the siren-disk alone rotates, cutting off and admitting the light by its rotation.

- Having now indicated the eneral outline of the apparatus which I pre er to use and which by anyone familiar with physical varied'in detail without departing from my invention, I' will now proceed to describe some of the precautions which I find it desirable to employ in order to effect the measurements to which I have referred with the greatest facility and precision. Obviously the invention will be employed although the arrangements which I am now about to indicate should be omitted; but the add so greatly to the precision of the resu t that in most cases it will be preferable to employ them.

I' have already'mentioned that the selenium cell should be inclosed in avacuum; the reason for this is that the seleniumis hygroscopic and when damp there is in such a "cell a certain electrolytic action between the selenium and its metallic support which gives rise to an electromotive force in the'cell itself, making it to a certain extent a primary'battery which may polarize, so that its E. M. F. varies between certain limits, thus rendering its determinations inaccurate by whatever amount "(if electric efiect may be present. In addition the slight heating of the cell causes it to give out gases, changing its internal resistance and again affecting its E. M. F.

- These deleteriousefiects are wholly avoided by inclosing the cell in a highly exhausted receiver and if in the course of this process a current be passed through the cell sufiicie'nt to heat it, at the same time continuing the ;action of the ump, it remains constant for a this construction of selenium cell, except in.

combination with other instrumentalities,

because it is describedand claimed, in my parallel pending application Serial Number 355,902, filed February 5th, 1907.

i The second precaution which should be employed is to construct the sup orting wire for the selenium of some materia which is not readily attacked by the metal, such as Y nickel, platinum, iron or German silver; nickel being the cheapest non-oxidizable, or substantially unoxidizable metal, is Well adapted to the purpose. Brass and copper wire may also be used; but they are objectionable because a selenite of copper is formed between which and the copper a smallE. M. F. maybe generated, im airing the accuracy and durability of the ce 1.

Another precaution which should be em-, ployed is to standardize the surface of emission of light so that the same extent of surface of the standard and of the light-source to be measured will be'exposed to the selenium cell.' One of the best methodsof accomplishing this is to screen the selenium from the light and pass the portion of light to be measured through a tube the inner surface ofwhich is blackened, of a definite area of cross-section, such as one square millimeter, or in some cases one square centimeter, though in general the latter unit will be found somewhat large. Obviously, llOW". ever, whatever .unit of surface be chosen should be the same for all of the lights which.

are tobe compared. Under some circumstances, as where the average horizontal and as the resistance of the selenium cell itself is very high, the total resistance in the circuit may be very large indeed. With a circuit of these characteristics, I have been .by'special a paratus enabled to start and stop large e ectncal machinery by passing my hand between the cell and the source of light Also for somewhat similar reasons the irolt-meter or 'galvanometer employed should be one of large resistance.

There are other methods of em loying measurable physical quantities for tlie purposes set out, and all of these which'employ theprinciples of m invention, I mm to cover 'by the claims 1n this and in ap lication already referred-to. avr us escri e 'myinven 1on,w a ll 'ng th d 'b d t h t I claim and wish. to. protect by Letters Patent .of the United States is v 1. In a light-measuring a paratus, the

combination of a source of lig t to be meas- In other.

means for measuring t e variation.

2. In an apparatus for measuring light,

means for causing the light to be measured to vary the current flow in an electric circuit, and means for measuring the variation.

3. In measuring apparatus for light, the combination of an electric circuit with a source of current therefor, a selenium cell interposed in the circuit, and means for measuring the variation of the electrical condition of the circuit caused b the action of a light to be measured upon t e seleniumcell.

4. The combination, in a light-measuring apparatus, of a source of current, an electric circuit, a selenium cell interposed therein, a voltmeter connected to indicate changes in the electrical condition of the circuit, and means for admitting the light to be measured to the selenium cell or excluding it therefrom.

5. In a light measuring apparatus, the combination of a source of current and an electric circuit including a selenium cell, with a volt-meter connected around the cell so as to measure the drop therein.

6. In a light measuring apparatus, the combination of a source of current and an electric circuit, with a selenium cell included in the circuit; a volt-meter connected across theterminals of the cell, and means for admitting the light to the cell or excluding it therefrom.

7-. In a light measuring apparatus, a source of current, a circuit composed in part of selenium, an electric measuring instrument connected in the circuit, and means for causing the light to act upon the selenium at desired times. i

8. In a light measuring apparatus, a

taining the selenium, and means for subjecting the selenium to the action of the light to be measured.

9. In a light measuring apparatus, a source of current, a circuit composed in part of a substance the electrical properties of which are res onsive to changes in illumination, means or measuring such responses, and means for admitting or excluding the light to be measured.

10. In a light measuring apparatus, a source of current, an electric circuit including selenium inclosed in a vacuum, an electric measuring instrument connected in the circuit, and means for causing the light to act upon the selenium at desired. times.

11. In a light measuring apparatus, a source of current, an electric circuit including a cell composed in part'of selenium inclosed in an exhausted receiver of quartz, an electric measuring instrument connected in the circuit, and means for causing the light to act upon the selenium at desired times.

12. In a light measurin apparatus, a source of current, an electr c circuit composed in part of selenium, the circuit being of high resistance, an electric measuring instrument connected in the circuit and means for causing the light to act upon the selenium at desired times.

13. In a light measuring apparatus, a source of current, a circuit composed in part of a substance the electrical properties of which are'responsive to chan es inillumination, means for measuring sucIl response, and means for exposing the selenium to or shieldin it from definite similar surfaces of the light to bemeasured and a standard light.

In Witness whereof I have hereunto set my name in the presence of two witnesses.

WILLIAM J. HAMMER.

Witnesses IRVING M. OLRIEGHT,

- ANITA BURKE. 

